This invention relates to a pneumatic method of converting ferrous metal to steel.
Pneumatic methods of producing steel from scrap and hot metal generally include blowing oxygen, air or mixture of oxygen and an inert gas, such as argon, into a metallic furnace charge for oxidizing such unwanted constituents as carbon, phosphorous and silicon. The oxygen or air can be delivered by tuyeres, the inner ends of which may be submerged or above the bath level. When submerged tuyeres are employed, they may be protected by a sheath of hydrocarbon shielding fluid injected in surrounding relation to the oxygen stream. It has also been suggested that oxygen may be introduced by tuyeres above the bath for the oxidizing of combustible off-gases whereby heat is added to the furnace charge. Such top tuyeres are shown, for example, in U.S. Pat. No. 3,839,017.
While conventional top-blown systems are satisfactory for the production of ordinary low-carbon steels, they are not wholly satisfactory. For example, bath mixing in the top-blown process is relatively poor in comparison to bottom blown systems. As a result, the iron content of the slag tends to be relatively high, that is, in the range of 15 to 30%. Such slags tend to foam resulting in considerable furnace slop and loss of iron from the system. As a result of these and other disadvantages, there have been attempts to convert top-blown systems into submerged tuyere furnaces.
One method for converting a top-blown metallurgical vessel to one having submerged tuyeres is discussed in U.S. Pat. No. 3,810,297 wherein conversion involves removing the furnace bottom and substituting a new bottom containing a plurality of two-pipe tuyeres. The inner pipes of such tuyeres are connected for delivering an oxygen stream to the molten metal bath while a concentric outer pipe is provided for delivering hydrocarbon shielding fluid. Also, the trunnion pins of such vessels are drilled for receiving oxygen and shielding fluid supply pipes which are connected to the respective tuyere pipes by connecting manifolds. As those skilled in the art will appreciate, it is also necessary in steel conversion methods to provide fluxing agents, such as lime, to the bath for desulfurization and phosphorous removal. This material is commonly entrained in the oxygen stream so that in bottom tuyere systems a lime distributor must be mounted on the lower end of the vessel so that the powdered material may be provided to each of the tuyeres. As a result of these process requirements, together with lime grinding, storage and injection equipment, conversion of a top-blown to a bottom-blown furnace is relatively expensive.